1. Field
The method herein relates to communication systems, and more specifically to a method for synchronizing communication system base stations via user equipment that operates on more than one communications standard.
2. Relevant Background
Base station synchronization has traditionally been accomplished via Global Positioning Systems (GPS) located in mobile stations or in fixed observation sites, or via terrestrial landline signaling. Landline signaling is typically poor in performance, and synchronization via GPS requires GPS availability and hardware. Base station synchronization is important for several purposes, including wireless user equipment (UE) position determination and for handoffs.
Global Navigation Satellite Systems (GNSS), including Global Positioning Systems (GPS) and/or GALILEO systems, offer an approach to providing UE position determination. A GNSS user can derive precise navigation information including three-dimensional position, velocity and time-of-day (TOD) through information gained from orbiting satellite vehicles (SVs).
Approaches other than GNSS are also used to determine position location in cellular networks. Several approaches perform triangulation based upon the use of timing information sent between each of several base stations and a UE device, such as a cellular telephone. In one approach, called Time Difference of Arrival (TDOA), the times of reception of a signal from a UE is measured at several base stations, and these times are transmitted to a location server, which computes the position of the UE. For this approach to work, the times-of-day at the various base stations need to be coordinated to provide accurate location. Also, the position of the base stations needs to be known accurately.
An alternative method referred to as estimated observed time difference (EOTD), measures at the UE the times of arrival of signals transmitted from each of several base stations. This timing data may then be used to compute the position of the UE. Such computation may be done at the UE itself or at a location server, if the timing information so obtained by the UE is transmitted to this server via the link. Again, the base station times-of-day must be coordinated and their location accurately assessed. In either approach, the locations of the base stations are determined by standard surveying methods and may be stored in the base station or at the server in some type of computer memory.
A combination of either the EOTD and TDOA and a GNSS system is called a “hybrid” system. In any of the above methods, time coordination between the various cellular base stations is necessary for accurate position calculation of the UE.
Accurate timing at base stations also provides UEs timing information for aiding GPS based position calculations; such information may result in reduced time to first fix, and/or improved sensitivity with respect to assisted GPS (AGPS) systems. The required accuracy for these situations can range from a few microseconds to around ten milliseconds depending upon the performance improvement desired. In a hybrid system, the base station timing serves the dual purpose of improving the time of arrival (TOA) or TDOA operation as well as the GPS operation.
In certain cellular systems, such as a Code Division Multiple Access (CDMA) cellular network, all base stations are equipped with GPS satellite receivers and TOD is known to all base stations and UEs operating in the network. All base stations in such systems are time synchronized. Other, asynchronous systems, such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), and Wideband Code Division Multiple Access (WCDMA), do not necessarily have time synchronized base stations and the base stations and UEs do not necessarily contain TOD information.